1. Field of the Invention
The present disclosure relates to a coating composition for a transparent electrode passivation layer and a transparent electrode having a passivation layer.
2. Description of the Related Art
Indium tin oxide (ITO) is generally used in transparent conductive films being currently used for transparent electrodes in the touch panel market of display industries. This is because ITO has excellent transparency in the entire visible light region, has a relatively low sheet resistance, and has an appropriate work function for injecting and collecting charge carriers in organic semiconductors. However, ITO has the following disadvantages: ITO is synthesized through a high temperature process, and thus cannot be used on a plastic substrate; ITO is very expensive; and ITO cannot be used for a flexible transparent conductive film due to its fragile nature.
Therefore, researches on materials which are capable of replacing ITO and have equivalent performances are being actively conducted.
Although there have been attempts to use a thin film made by sputtering a metal, such as gold, silver, and copper, as a transparent electrode, there are problems in that the thin film has a low light transmittance in the visible light region while having excellent electrical conductivity, and exhibits poor adhesion with a lower substrate. Furthermore, there are problems in that a ZnO thin film has an electrical conductivity lower than that of ITO while being an inexpensive material, and an ATO thin film, in which a small amount of Sb is added to SnO2, is difficult to etch and has a high sintering temperature.
Furthermore, although a method for producing an oxidation film using sol-gel synthesis is being used, the method is problematic in that the electric conductivity is still low and a high temperature process in which the sintering temperature is above 350° C. is required.
Thus, researches for applying metal nanowires, particularly silver nanowires having excellent electrical conductivity and thermal conductivity, to a transparent electrode are being actively conducted.
Metal nanowires serve to ensure electrical conductivity by forming a network during forming of a transparent electrode. As the metal nanowire network is more densely formed, the electrical conductivity of the transparent electrode is improved. However, there are problems in that the visible light transmittance of the transparent electrode is reduced and excessive costs are required. Even if a conductive network is formed using metal nanowires, the metal nanowires are broken due to thermodynamic instability even under the condition of only a temperature of about 200° C., and electrical and optical properties thus drastically deteriorate. That is, such a network disconnection inevitably occurs, and empty spaces between the networks remain as non-conductive regions having no conductivity. Furthermore, the metal nanowires as a nanostructure are more active than conventional materials, and thus highly prone to oxidation and corrosion when exposed to the atmosphere without a passivation layer. Particularly, while silver nanowires have a highly conductive property and are transparent in the visible light region, it is known that the resistance increases by about 15 to 20% due to the oxidation and corrosion in the atmosphere. Thus, there was a problem in that additional antioxidants or a plurality of passivation layers were required in order to prevent the oxidation and corrosion.
In order to improve the heat resistance and durability of such metal nanowires, researches for using an oxide (ZnO, IZO, or AZO), graphene, carbon nanotube (CNT), or the like as a passivation layer are being conducted.
As a specific example, D. S. Ghosh et al. suggest zinc oxide (ZnO) as a passivation layer of a transparent electrode including a metal nanowire layer (Applied physics letters, 102, 221111, 2013). Also, Donghwa Lee et al. suggest graphene as a passivation layer of a transparent electrode including a metal nanowire layer (nanoscale, 5, 7750, 2013). However, conventional passivation layers lead to a decrease in the transmittance of a transparent electrode including a silver nanowire layer.
Also, KR Patent Publication No. 10-2014-0094415 discloses a coating composition for a transparent conductive film, a transparent conductive film, and a method for fabricating the same. The prior art literature discloses a magnesium fluoride sol, an inorganic sol, an inorganic-inorganic composite sol, and an organic-inorganic hybrid sol as a coating composition for a passivation layer for protecting a metal nanowire electrode. However, the coating layer for a passivation layer disclosed in the literature has a problem of insufficient heat resistance and hardness.
Thus, while investigating a passivation layer capable of ensuring both the electrical conductivity and the light transmittance of a transparent electrode including a metal nanowire layer, the present inventors developed a coating composition for a transparent electrode passivation layer, the coating composition including a metal oxide and a specific compound of ethylene glycol or the like. The present inventors found that when a passivation layer was prepared using the coating composition for a transparent electrode passivation layer and was applied to a transparent electrode, the passivation layer was capable of ensuring the heat resistance and durability of the transparent electrode while maintaining the transmittance of the transparent electrode, and finally completed the present invention.